Subsurface Structure of Singkarak Pull Apart Inferred from Magnetic and Gravity Modeling

The existence of active faults along the Sumatran fault has an impact on tectonic activities such as earthquakes and the formation of pull-apart along the Great Sumatran Fault (GSF). The Great Sumatran Fault is segmented into 19 segments. Fourteen segments of which are pull-apart. One of the pull-apart amongofthe GSF is the Singkarak pull-apart which is located on the Sumani segment. Magnetic and gravitational methods are appropriate geophysical methods used to determine rock structures below the earth’s surface. The subsurface structure of the Singkarak pull-apart analysis needed to identify potential future basins. Based on the analysis of the low magnetic anomaly closures on both sides of Lake Singkarak, this study has also succeeded in identifying the presence of a less dense Quaternary Coluvium volcanic deposition outcrop and concluded that the displacement of about 18± 3 km was resulted from a strike-slip process in the past. Furthermore, Lake Singkarak is estimated to have experienced an opening as far as 8 ±1 km which is based on the maximum width of the model profile that passes through the middle of Lake Singkarak.


Introduction
The subduction between the Indian-Australian plate under the Eurasian plate on the island of Sumatra occurs obliquely [1][2][3][4][5][6].This has an impact on the emergence of tectonic activity in the form of faults and magmatism.The Sumatran Fault (Great Sumatra Fault/GSF) has a length of 1900 km and is segmented into 19 segments [7][8].Tectonic activity causeof GSF along the island of Sumatra is a dextral strike-slip fault system [8][9][10][11].The existence of this dextral strike-slip fault cause a pull-apart basin in several segments.Among them is the Singkarak pull-apart basin in the Sumani and Sianok segments which form a Singkarak tectonic lake in West Sumatra [8][9].Geomorphologically, the Singkarak pull-apart basin in the Sumani and Sianok segments can be easily recognized in its surface appearance as lowlands, mountains, and even lakes.However, the underlying structure is frequently overlooked.Knowledge of subsurface conditions is crucial in hazard mitigation, especially earthquakes, and understanding another hazard potential related to the pull-apart basin.The Singkarak pull-apartment is one of 14 pull-apart basins along the island of Sumatra [12].Thus, studying the subsurface geometry of the Singkarak pull-apart may help us better understand the characteristics of other pull-apart basins along the GSF.
A ground magnetic survey was carried out for the first time around the Sumani segment to study the underlying structure.Geomagnetic observations are thought to be useful since the study area is igneous rocks dominated, which are expected to show a robust magnetic response.[13].The magnetic method in this study was combined with regional gravity data [14][15].The combined magnetic and gravity approaches have long been successfully used to determine the geometry of the subsurface in many places [16][17][18][19], and may provide valuable information [20][21].

Geological Setting
The geological research area consists of rocks ranging from Permian to Quaternary.The Permian metamorphic rocks as shown in Figure 1 (Pq, Ps, Pl, Pcks, and Pckl) shared similarity in composition and age and can be considered as a singleentity (Pq) to simplify discussion.The Tertiary age rock units discovered in the study area are the Miocene intrusive granite units (Tmgr, g).The presence of many intrusive rock out crops in this location is unsurprising, given that it is located along a volcanic route caused by Sumatra's subduction process.Limestone conglomerates (TRtl), slateshale (TRts), quartz sandstone deposits containing arkosemica (Tmol) and marlshale deposits (Tos) are other Tertiary rock outcrops found in this area.Quaternary rocks are found as volcanic deposits of colluvium (QTau), Marapi andesite (Qama; -500 masl), alluvium surface deposits (Qal; -200 masl) and Singgalang Tandikat andesite (Qast).In general, the rock units found in the Sumatran fault in the Sumani segment are more dominated by Quaternary young volcanic rocks (0-1.6Ma)(QTau and Qama) followed by other rock units of Tertiary age (1.6-66Ma) ( Tmgr, TRtl and TRts), Triassic (205-240Ma) (g) and Permian (240-290Ma) (Pcks, Pckl, Pq and Pl).All descriptions of the rock units described above can be seen in the combined geological map ofthe Padang sheet [22] and the Solok sheet [23] (see Figure 1).Figure 1.Geological map around Sumani segment, lithology and fault lines (modified from [22][23]).

Data and Methods
This study employs a geomagnetic method, and 2 units of GEM Proton Precision Magnetometer GSM-19T (GEM system, Canada) were used.Geomagnetic surveys were carried out in the following areas in West Sumatra Province (Agam, Batusangkar, Bukittinggi, East Pasaman, Padang, Padang Panjang, Padang Pariaman, Solok, and Tanah Datar).The Measuring points were distributed on highway sand gravel route saccessible by motorcycle with 500-1000 meter intervals varying depending on the distance to the fault line.
The raw magnetic data from the field survey was then processed with various corrections, including diurnal correction and IGRF (International Geomagnetic Reference Field) correction, to obtain the value of magnetic intensity.The gravity data obtained from the Bouguer anomaly maps of Solok and Padang sheets were used [22][23] and digitized to complement the magnetic data.We used GM-SYS Oasis Montaj (Geosoft Inc., Toronto, Canada) to further process the magnetic and gravity data.The minimum curvature approach was used to gridding point data with a blanking distance of 5000 meters.Since the magnetic intensity map shows some local bulls-eye anomaly, the Gaussian regional filter is used to smoothen the magnetic intensity map.The filtering process uses the inclination and declination values of -18° and -0.5 °, respectively, representing the magnetic properties of the area.

Result and Discussion
This study has been identified regions with low to high magnetic anomalies (Figure 2).Then to see the perspective of magnetic and gravity anomalies, a three-dimensional profile was made below the surface of the Singkarak pull apart in the Sumani segment as shown in the fence diagram (Figure 3).From the modeling results, it can be seen that there are unxpose bodies, namely bodies of intrusive rock and metamorphic sliver.The body of this intrusive rock may have been a place of accumulation of stress forces (locking zone) before the release of energy into an earthquake in that place.The presence of a low magnetic anomaly located at the northwest end and in the southeastern part of Lake Singkarak (see Figure 4) is thought to be related to the magnetically weak colluvium volcanic deposit (Qtau).The presence of sediments across the lake indicates that these sediments were previously a single body that was later separated and shifted due to the tectonic deformation that formed Lake Singkarak.This interpretation is summarized in Figure 5.It was previously thought that this rock formation was a single unit, but due to the pull-apart, these rock units were separated from each other 18  3 km with uncertainty in determining the QTau marker estimated at 3 km.Meanwhile, the opening of Singkarak Lake is 8 1 km based on the estimated maximum width of Singkarak Lake.If it is related to the slip rate of the Sumatran fault in Central Sumatra based on data from the Global Positioning System (GPS), it shows that the slip rate varies around 20 mm/year [3].So, for rocks that are quaternary (0.7 to 1.8 Ma) the displacement value of 18  3 km is within the range of possible values.These results can strengthen the results of previous studies which stated that the displacement pull-apart Singkarak is 23 km with an opening width of 8 km [7].The shift value of 18 km in this study is not the distance from one end to the other of Lake Singkarak (see Figure 5) as suggested by Sieh and Natawidjadja (2000), but rather from the approximate distance of the presence of low magnetic anomaly closures at two locations on both sides of the lake.Singkrak represented by the rock unit Qtau.This study looks at the displacement due to the Singkarak pull-apart from the other side, namely from the magnetic anomaly data which is expected to represent the formation of a lake, namely Lake Singkarak in the past.Besides, the results of this study can also strengthen the results of research by Bellier and Sebrier (1994) which states that the right step over pull-apart from Lake Singkarak is 21 km long and 7 km wide (030' to 040'S).

Conclusions
The results of the analysis of low magnetic anomaly closures on both the southwest and southeast sides of Lake Singkarak are thought to be related to the presence of less dense Quaternary colluvium volcanic deposits as a result of the earlier pull-apart strike-slip process in Singkarak.It is estimated that these rock formations are 18  3 km apart from each other with uncertainty in determining the Qtau marker estimated at 3 km.Lake Singkarak is estimated to have experienced an opening as far as 8  1 km which is based on the estimated maximum width of Lake Singkarak which is on both sides of Lake Singkarak.Under the surface of the Singkarak pull-apart, it is estimated that there are also unexposed intrusive bodies that may act as a locking zone where earthquake stress energy accumulates and eventually causes an earthquake.In addition, in this area there may also be the presence of metamorphic sliver.

Figure 2 .
Figure 2. Magnetic intensity map in the area around the Sumani segment.The thick blue line indicates the profile line, the thin black line indicates the fault while the thick red line indicates the active fault.The thick blue line shows the boundary of Lake Singkarak.

Figure 3 .
Figure 3. Fence diagram based on the results of subsurface modeling of magnetic anomalies and gravity from Singkarak pull apart.

Figure 4 .
Figure 4. Low magnetic anomalies on the northwest and southeast sides of Lake Singkarak correlate with the presence of the Qtau formation outcrop which is thought to be separated due to the Singkarak pull-apart.

Figure 5 .
Figure 5. Sketch of rock deformation due to pull-apart around Lake Singkarak Further explanation on the study of subsurface structures on the Sumatran fault in the Sumani segment can be seen in the journal Tectonophysics which has been published on November 15, 2021 [24].